1. Technical Field
The present invention generally relates to a sensor for detecting the presence of moisture. More particularly, the present invention is directed to an optoelectric sensor for detecting the presence of moisture and/or rain on the outside surface of a variety of energy modifying glass windshields.
2. Discussion
Rain sensors that rely on light or infrared (IR) energy reflecting off of the outside surface of a windshield operate under the well understood phenomenon of total internal reflection, and are generally known within the art. In a typical vehicle configuration, the rain sensor is electrically interconnected with the windshield wiper control circuit. Thus, when the presence of moisture or rain is detected on the windshield surface, a signal can be generated for triggering operation of the windshield wipers. These conventional rain sensors also have the capability of detecting the intensity of rain accumulation and may in turn control the windshield wiping frequency.
With reference to FIG. 1, a conventional rain sensor 10 which is optically coupled to a solar absorptive glass windshield 20 is shown. Rain sensor 10 is shown as including an energy source or emitter 12 which emits light energy 28, and using an optical lens 16, directs this energy through inside surface 24, and toward the outside surface 26 of the windshield 20 at an incident angle of principally 45 degrees. This light energy 28 is reflected at the outside surface 26, back through the inside surface 24, and focused by an optical lens 18 onto a photo sensitive detector 14, such as a photo transistor or photo diode. The presence of moisture or rain on the outside surface 26 of the windshield causes a change in the angle of reflection of the incident light energy 28. This change in the angle of reflection results in less light energy 28 being reflected back to the photodetector 14. The electronics controlling rain sensor 10 are capable of detecting moisture and/or rain 22 on the outside surface 26 of the windshield 20 by monitoring and analyzing the amount of light energy 28 returning to photodetector 14.
Advances in windshield technology have allowed the introduction of high performing glasses, such as IR reflective glass. A cross section of this type of IR reflective glass 30 is schematically represented in FIG. 2, and is shown to include an inner reflective layer 32 made up of a material that reflects energy in the IR spectrum. The inner reflective layer 32 is typically a microscopic layer of silver or other suitable reflective material which is situated between an inner glass layer 33 and an outer glass layer 35. This type of windshield glass is highly reflective at IR wavelengths, which assists in keeping the interior of the vehicle cooler when subjected to sunlight. At the same time, this IR reflective glass has a transmissivity level of greater than 75% of the visible spectrum. The transmissivity level of IR reflective glass is typically greater than that of solar absorbing glass in the visible spectrum. Thus, IR reflective glass is favored in many automotive markets since government regulations will not allow solar absorbing glasses to be used because they do not meet the regulated transmissivity levels for visible light.
With continued reference to FIG. 2, this inner reflective layer 32 of windshield 30 creates a significant challenge for IR based rain sensors, such as rain sensor 10, because the inner reflective layer 32 tends to reflect a large amount of the incident light energy 28 to the photodetector 14 before it reaches the outside surface 36 of the glass. The light energy 28 reflected from outside surface 36 is represented as dashed line ray traces 39, and the light energy 28 reflected from inner layer 32 is represented as solid line ray traces 38, both illustrated in FIG. 2. This reflection of light energy 38 from reflective layer 32 reduces the sensitivity and effectiveness of the rain sensor 10 because a larger percentage of the incident energy 28 transmitted by the emitter 12 is reflected from the inner reflective material layer 32 and not the target area of outside surface 36. For example, in a typical IR reflective windshield having an inner reflective layer 32, calculations show that this inner reflective layer causes a sensitivity reduction of the sensor of over 28 dB. Thus, a rain drop 22 landing on the windshield's outside surface 36 has a smaller effect on the change in total energy seen by the photodetector 14.
This change could be compensated for by means of electrically amplifying the signal or by changing multiplying factors in the control and analysis software. However, these methods are undesirable in that a sensor which is modified to work on reflective glass, such as IR reflective glass 30 would be too sensitive on non-reflective or solar absorptive glass, such as windshield 20. Alternatively, separate sensors would need to be incorporated within rain sensor 10 to detect the inner reflective layer 32 allowing the control and analysis software to adapt or switch between operating modes. However, this method adds complexity and cost to the system.
Additionally, this problem is difficult to solve using only an electrical or electronic approach because of the already high gain levels used in the circuitry of these rain sensors. Moreover, if a rain sensor is customized for a particular type of windshield, there is no assurance that the vehicle will not have its windshield replaced in the future with a different type of windshield, thus causing unknown results, including the rain sensor not working on the glass at all. Accordingly, a contemplated solution is to modify the windshield optical coupling device associated with the rain sensor, which is typically supplied with the windshield. To this end, the problems associated with glass replacement and customizing rain sensors for particular windshield reflective layer characteristics are eliminated.
In view of these problems, it is desirable to provide a device and technique for minimizing the effects of the reflective properties of the inner reflective layer associated with IR reflective windshields. In addition, it is desirable to create a rain sensing system that has similar performance using the same sensor on a variety of IR reflective and solar absorptive glass windshields, requiring only a different optical attachment coupler to be bonded to the windshield. It is also desirable to provide an electronic rain sensor system in which a common optoelectric configuration can be used with both IR reflective and solar absorptive glass windshields. Furthermore, it is desirable that this common optoelectric configuration work on a variety of IR reflective glass windshields having different transmissivity levels. Such a device would allow the same rain sensor to be used on a replacement windshield having different reflective properties or physical characteristics without recalibrating the sensor. Finally, it is desirable to provide an optical attachment coupler which is designed for a specific windshield curve, which also includes a standard mounting configuration for receiving and securing the rain sensor in optical contact with the inside surface of the windshield.